The present invention relates to a transfer device and an image formation apparatus such as a copier, a facsimile, or a printer each equipped with the transfer device. More particularly, this invention relates to the transfer device which has a transfer belt supported by a plurality of rotors and carries and conveys a transfer material, and a cleaning member that cleans the surface of the transfer belt, and which transfers a toner image formed on the image carrier onto a transfer material on the transfer belt. This transfer device also has a dielectric layer on the surface of the cleaning member. This invention also relates to the image formation apparatus with the transfer device.
Conventionally, a transfer device for transferring a toner image formed on an image carrier onto a transfer material has been provided in an image formation apparatus such as a copier or a printer.
As this type of transfer device, there has been known one that carries transfer paper as a transfer material on the transfer belt and conveys it to a transfer area where a photosensitive body as an image carrier and a transfer belt supported by a support roller as a rotor are provided opposite to each other and contact each other. This transfer device then generates a transfer electric field at the transfer area when the transfer paper passes through the transfer area, and transfers the toner image formed on the photosensitive body onto the transfer paper by this transfer electric field.
In the transfer device configured as explained above, because the transfer belt and the photosensitive body contact each other via transfer paper, toner dirt stuck on the photosensitive body or splashed toner at the time of transfer maybe deposited on the transfer belt, which causes inconvenience such as soil on the back of the transfer paper or defective transfer. Therefore, it is necessary to additionally provide a cleaning unit to remove the toner deposited on the transfer belt in the transfer device.
As a cleaning unit for a transfer belt, there has been conventionally known a cleaning unit in an electric-field cleaning method in which the toner on the transfer belt is electrostatically moved to the side if a cleaning member by rubbing the surface of the transfer belt with a conductive member as a cleaning member to which a cleaning bias of the opposite polarity to an electric charge polarity of the toner is applied (e.g., JP, HEI 07-64444 A, JP, HEI 09-152788 A). In this electric-field cleaning method, an electric field which has electrostatic force (hereafter referred to as bias electric field) to allow the toner on the transfer belt to move toward the conductive member is produced between the conductive member and the transfer belt. The toner on the transfer belt is moved to the cleaning member side by this bias electric field to clean the toner off the transfer belt.
In the electric-field cleaning method, however, sufficient cleaning performance could not be obtained, and imperfect cleaning sometimes occurred. This imperfect cleaning easily occurs especially when a transfer belt with high resistance such that volume resistivity is 1012 to 1013 ohm-cm or more is used or when the amount of charge on toner is large.
When the high-resistance transfer belt is used, force with which toner is electrostatically absorbed to the transfer belt is increased because dielectric polarization is easily generated as compared to a belt with low resistance, and transfer charge applied to the belt at the time of transfer, that is, electric charge of the opposite polarity to an electric charge polarity of toner is easily held on the charge-applied surface. When the amount of charge on the toner is large, the force with which the toner is electrostatically absorbed to the transfer belt is also increased based on Coulomb force or image force. The toner on the transfer belt is layered, therefore, when electrostatic absorption force between the toner and the transfer belt is such strong, there is sometimes a case where even the toner on the top layer can not be moved to the conductive member side by the cleaning electric field. Therefore, it is conceivable that, when the electrostatic absorption force between the toner and the transfer belt is strong, a cleaning bias to be applied to the conductive member is increased in order to obtain excellent cleaning performance according to the electric-field cleaning method.
However, when the cleaning bias is too large, dielectric breakdown may be produced at a contact section between the cleaning member and the transfer belt in the thickness direction of the transfer belt, which may cause a current to leak. When the current leaks, the bias electric field is not formed, thus, sufficient cleaning performance in the electric-field cleaning method can not be attained.
Further, when the cleaning bias is too large, electric charge of the opposite polarity to that of toner is injected to the toner on the transfer belt, so that the electric charge polarity of the toner may be reversed. Such toner whose electric charge polarity has been reversed (hereafter referred to as reversed-polarity toner) can not be cleaned off in the electric-field cleaning method. Therefore, the reversed-polarity toner becomes a cause of reduction in cleaning performance.
Not only is there a case where the reversed-polarity toner is generated by injection of charge, but also there is a case where the reversed-polarity toner may be deposited on the transfer belt when the reversed-polarity toner existing on the surface of the photosensitive body contacts the transfer belt. Such reversed-polarity toner then becomes a cause of reduction in cleaning performance as well.
The applicant of this invention has proposed (Patent Application No. HEI 11-301773) a transfer device which can clean the surface of a transfer belt more sufficiently than the conventional electric-field cleaning method and also can clean reversed-polarity toner off the belt even when a high-resistance transfer belt is used or when the amount of charge on toner is large, and also has proposed an image formation apparatus with the transfer device. In this transfer device, when the dielectric layer provided on the surface of the cleaning member and the toner on the transfer belt contact each other, electric charge of the opposite polarity to an electric charge polarity of the toner is induced on the surface of the dielectric layer by dielectric polarization, and electrostatic absorption force is generated between the dielectric layer and the toner. Accordingly, electrostatic absorption force between the toner and the dielectric layer (hereafter referred to as dielectric absorption force) and electrostatic absorption force between the transfer belt and the toner (hereafter referred to as belt absorption force) act on the layered toner between the transfer belt and the dielectric layer. Each magnitude of the respective electrostatic absorption force acting on the toner varies inversely with the square of the distance to the toner. Therefore, the dielectric absorption force is stronger than the belt absorption force with respect to the toner on the top layer of the surface of the transfer belt, which allows the toner to be moved to the side of the dielectric layer and the toner to be cleaned off the transfer belt. Accordingly, even when a high-resistance transfer belt is used or when the amount of charge on toner is large, the surface of the transfer belt can be more sufficiently cleaned as compared to that of the conventional electric-field cleaning method. Further, electric charge of the opposite polarity to an electric charge polarity of toner is always induced on the surface of the dielectric layer, therefore, the toner whose electric charge polarity has been reversed can also be moved to the surface of the dielectric layer. Resultantly, soil on the back of a transfer material due to imperfect cleaning of a transfer belt or defective transfer do not possibly occur.
It has also been proposed in the specification according to the earlier filed application that a bias application unit, that applies a cleaning bias of the opposite polarity to the electric charge polarity of the toner to the cleaning member, is provided and also a bias electric field which allows the toner to move toward the cleaning member is produced between the cleaning member and the transfer belt. According to the specification, a cleaning effect of an electric field is added to a cleaning effect of a dielectric layer, which allows more excellent cleaning performance to be attained.
A first object of this invention is to provide a transfer device, which can maintain a cleaning effect of a dielectric layer higher than a predetermined level and effectively clean a transfer belt by specifying characteristic values of the dielectric layer provided on the surface of a cleaning member, and an image formation apparatus with the transfer device.
A second object of this invention is to provide a transfer device, which can maintain a cleaning effect of a dielectric layer higher than a predetermined level and effectively clean a transfer belt by specifying a value of electrostatic capacitance of the dielectric layer provided on the surface of a cleaning member, and an image formation apparatus with the transfer device.
A third object of this invention is to provide a transfer device, which can maintain a cleaning effect of a dielectric layer on a transfer conveyor belt higher than a predetermined level and effectively clean the transfer conveyor belt by defining a material of the dielectric layer provided on the surface of a cleaning member, and an image formation apparatus with the transfer device.
In the earlier application, there has been disclosed the example of a configuration when a cleaning roller is used as the cleaning member and the outer periphery of the roller is coated with a tube made with a nylon-base material as the dielectric layer. When the cleaning roller is thus coated with the tube, the tube may not follow rotation of the roller and slip if adhesion between the two is not sufficient. When the slip caused by such insufficient adhesion occurs, imperfect cleaning of a transfer conveyor belt occurs.
In order to keep adhesion between the tube and the roller, it is also conceivable to bond the two with an adhesive. However, application of the adhesive makes these two awkward to be handled at the time of manufacturing. Further, depending on the adhesives, electric nonuniformity occurs on the cleaning roller. These disadvantages should be avoided.
A fourth object of this invention is to provide a transfer device which can prevent imperfect cleaning of a transfer conveyor belt due to insufficient adhesion by promoting adhesion between a roller-shaped member and a dielectric layer without using an adhesive when the dielectric layer is provided on the roller-shaped member as a cleaning member, and an image formation apparatus with the transfer device.
According to one aspect of this invention, in a transfer device, a part of toner on a transfer belt moves to the surface of a dielectric layer by dielectric polarization generated on the surface of the dielectric layer due to a contact between the dielectric layer and the toner. While some other part of the toner moves to the surface of the dielectric layer by a bias electric field formed by a bias application unit. At this time, if volume resistivity of the dielectric layer is too small, the dielectric layer exhibits conductivity. Therefore, the dielectric polarization due to a contact of the layer to the toner hardly occurs, a cleaning effect of the dielectric layer is reduced, thus, only a cleaning effect of the electric field may be obtained. Further, if a cleaning bias to be applied to a cleaning member is increased in order to enhance the cleaning effect of the electric field when the volume resistivity of the dielectric layer is too small, dielectric breakdown may occur in the thickness direction of the dielectric layer.
Therefore, in this transfer device, by setting volume resistivity of the dielectric layer to 1E3 ohm-cm or more, the conductivity of the dielectric layer is not made too high, so that the cleaning effect of the dielectric layer is maintained higher than a predetermined level. Further, the dielectric breakdown of the dielectric layer due to the cleaning bias is prevented.
Further, if the volume resistivity of the dielectric layer is too large, insulating strength of the dielectric layer becomes too high. Accordingly, a bias electric field produced between the cleaning member and the transfer belt becomes insufficient, which may cause the cleaning effect of the electric field to be reduced.
Therefore, in this transfer device, the volume resistivity of the dielectric layer is set to 1E15 ohm-cm or less, so that the bias electric field can be ensured sufficiently and the cleaning effect of the electric field can also be maintained higher than a predetermined level.
Further, in this transfer device, a cleaning bias to be applied to the cleaning member is set to 300 volts or more, so that the cleaning effect of the electric field is sufficiently ensured. The cleaning bias is also set to 2500 volts or less, so that the cost of power to be used will not increase.
According to another aspect of this invention, surface resistivity of the dielectric layer is set to 1E4 ohms/square or more, so that the conductivity of the dielectric layer is not made too high, and the cleaning effect of the dielectric layer is maintained higher than a predetermined level.
According to still another aspect of this invention, in a transfer device, if a dielectric constant of the dielectric layer is too small, electrostatic capacitance becomes too small, so that the amount of charge, that is, the amount of toner which can be deposited on the surface of the dielectric layer is decreased, and the cleaning effect of the dielectric layer may be reduced.
Therefore, in this transfer device, the dielectric constant of the dielectric layer is set to 3 or more, so that the sufficient electrostatic capacitance is stored, and the cleaning effect of the dielectric layer is maintained higher than a predetermined level.
Further, in this transfer device, the cleaning effect of the dielectric layer is maintained higher than a predetermined level and further the cleaning effect of the electric field can be obtained. Thus, the surface of the transfer belt can be cleaned more sufficiently.
According to still another aspect of this invention, in an image formation apparatus, the transfer device is used as a transfer device, so that toner dirt stuck on an image carrier which is deposited on the transfer belt or splashed toner at the time of transfer can sufficiently be cleaned off in a state where the cleaning effect of the dielectric layer is maintained higher than a predetermined level. Thus, soil on the back of a transfer material due to imperfect cleaning of the belt, or defective transfer can be prevented from its occurrence.
According to still another aspect of this invention, in a transfer device, toner on a transfer belt moves to the surface of a dielectric layer by dielectric polarization generated on the surface of the dielectric layer due to a contact between the dielectric layer and the toner. At this time, if the electrostatic capacitance of the dielectric layer is too small, the amount of charge, that is, the amount of toner which can be deposited on the surface of the dielectric layer is decreased. Thus, the cleaning effect of the dielectric layer may be reduced.
Therefore, in this transfer device, the electrostatic capacitance of the dielectric layer is set to 100 pF or more, so that the amount of charge enabling deposition on the surface of the dielectric layer is sufficiently stored, and the cleaning effect of the dielectric layer is maintained higher than a predetermined level.
The value of electrostatic capacitance is determined by a dielectric constant and a thickness of the dielectric layer. Therefore, there is a conceivable method here for controlling the dielectric constant by dispersing, for example, a resistance control agent into the dielectric layer to manage the electrostatic capacitance. However, the volume resistivity and the surface resistivity of the dielectric layer vary in response to variation of the dielectric constant, which is not preferable. Therefore, it is desirable that the electrostatic capacitance is managed by changing the layer thickness of the dielectric layer. According to this method, the electrostatic capacitance can be set to any amount of electrostatic capacitance so as to obtain a cleaning effect higher than a predetermined level without changing of the other characteristic values.
Further, in the transfer device, the cleaning effect of the dielectric layer is maintained higher than a predetermined level and further the cleaning effect of the electric field can be obtained, thus more sufficiently cleaning the surface of the transfer belt.
Further, in the transfer device, the cleaning bias to be applied to a cleaning member is controlled to 300 volts or more, so that the cleaning effect of the electric field is sufficiently ensured. Further, by controlling the cleaning bias to 2500 volts or less, the cost of power to be used will not increase.
According to still another aspect of this invention, in an image formation apparatus, the transfer device is used as a transfer device, so that toner dirt stuck on an image carrier which is deposited on the transfer belt or splashed toner at the time of transfer can sufficiently be cleaned off in a state where the cleaning effect of the dielectric layer is maintained higher than a predetermined level. Thus, soil on the back of a transfer material due to imperfect cleaning of the belt, or defective transfer can be prevented from its occurrence.
According to still another aspect of this invention, a transfer device comprises a transfer conveyor belt which carries and conveys a transfer material, and a cleaning member which contacts the transfer conveyor belt to clean the surface of the belt. Further, this transfer device transfers a toner image formed on an image carrier onto the transfer material on the transfer conveyor belt. A dielectric layer is provided on the surface of the cleaning member, and surface roughness of the dielectric layer is made equal to or less than a particle diameter of toner.
According to still another aspect of this invention, in a transfer device, toner on a transfer belt moves to the surface of a dielectric layer by dielectric polarization generated on the surface of the dielectric layer due to a contact between the dielectric layer and the toner. In such a transfer device, even when a transfer conveyor belt with high resistance is used or when the amount of charge on toner is large, the surface of the transfer conveyor belt is more sufficiently cleaned as compared to that of the conventional electric-field cleaning method, and the reversed-polarity toner can also be cleaned off.
Especially, surface roughness Rz of the dielectric layer is set so as to be equal to or less than the particle diameter of the toner. According to the experiment by the inventors of this invention, when a cleaning member with such a dielectric layer is used, cleaning performance on the transfer conveyor belt by the dielectric layer can be maintained more stably and higher than a predetermined level. This is because a contact portion between toner and the dielectric layer becomes larger as compared to the case where the surface roughness Rz of the dielectric layer is larger than the toner particle diameter, which makes it possible to move a larger amount of toner to the surface of the dielectric layer.
By the way, the applicant of this invention has proposed the transfer device, as the invention of improvement according to the earlier filed application (Patent Application No. HEI 11-301773), which can maintain a cleaning effect of a dielectric layer higher than a predetermined level and effectively clean a transfer conveyor belt by specifying characteristic values of the dielectric layer, and an image formation apparatus with the transfer device (Patent Application No. HEI 11-345064 and Patent Application No. HEI 11-345069). In the transfer device and the image formation apparatus according to Patent Application No. HEI 11-345064, a dielectric layer whose dielectric constant is 3 or more is provided on a cleaning member. In the transfer device and the image formation apparatus according to Patent Application No. HEI 11-345069, a dielectric layer whose electrostatic capacitance is 100 pF or more is provided. By providing a dielectric layer with such characteristics, the amount of charge enabling deposition on the surface of the dielectric layer is sufficiently stored, and a cleaning effect of the dielectric layer can be maintained higher than a predetermined level.
The dielectric constant is a constant determined by the material of a dielectric layer, and the electrostatic capacitance is determined by the dielectric constant and the layer thickness of the dielectric layer. Therefore, the inventors of this invention have carried out studies on a layer thickness with which electrostatic capacitance satisfies the range when a dielectric layer made with a nylon-base material whose dielectric constant is 11 is used. As a result, it has been clear that if the layer thickness of the dielectric layer is 1000 xcexcm or less, the amount of charge enabling deposition on the surface of the dielectric layer can sufficiently be stored and the cleaning effect of the dielectric layer can also be maintained higher than a predetermined level.
Therefore, in this particular transfer device, a dielectric layer formed with a nylon-base material whose dielectric constant is 11 and whose layer thickness is 1000 xcexcm or less is provided. By using such a dielectric layer, the amount of charge enabling deposition on the surface of the dielectric layer can sufficiently be stored, and the cleaning effect of the dielectric layer can be maintained higher than a predetermined level.
Since durability of the dielectric layer is reduced by making the layer thickness of the dielectric layer thinner, it is not desirable to make the layer thickness too thin. According to the experiments by the inventors of this invention, it has been ascertained that the durability of the dielectric layer becomes insufficient when a dielectric layer made with the nylon-base material with its dielectric constant of 11 is used and its layer thickness is set to 60 xcexcm or less. However, the layer thickness of the dielectric layer can be made thinner by improving the durability by taking measurements for abrasion resistance of the dielectric layer.
According to still another aspect of this invention, in a transfer device, even when a transfer conveyor belt with high resistance is used or the amount of charge on toner is large, the surface of the transfer conveyor belt can be cleaned more sufficiently as compared to the conventional electric-field cleaning method and reversed-polarity toner can also be cleaned off the belt by the same effects as those of the transfer device as explained above.
In this transfer device, by press-fitting a roller-shaped member into a hollow section of the dielectric tube, adhesion between the two can be promoted without using an adhesive. Therefore, the dielectric tube does not possibly slip at the time of rotation of the roller-shaped member, thus preventing imperfect cleaning of the transfer conveyor belt due to such slip.
Even in the transfer device, mechanical characteristics of the dielectric tube vary according to variations in temperature or humidity and a high degree of adhesion between the dielectric tube and the roller-shaped member can not be maintained, therefore, the possibility that the dielectric layer slips may increase.
Further, in this transfer device, in order to solve the problem, movement of the dielectric tube is restricted in the axial and peripheral direction of the roller-shaped member by tapered extending sections. Accordingly, the possibility that the dielectric tube slips can be decreased more reliably. Thus, occurrence of the slip can be suppressed even when the adhesion between the dielectric tube and the roller-shaped member decreases due to variations in temperature or humidity.
Further, in this transfer device, a bias electric field which allows toner to move toward a cleaning member is produced by the bias application unit between the cleaning member and the transfer conveyor belt. Therefore, a cleaning effect of the bias electric field can also be attained in a state where cleaning performance on the transfer conveyor belt by the dielectric layer is sufficiently maintained. Thus, the surface of the transfer conveyor belt can be more sufficiently cleaned.
Especially, when the bias application unit is provided in the transfer device, because the surface of the dielectric layer provided on the surface of the cleaning member is comparatively smooth, density of a line of electric force in the bias electric field near the surface of the dielectric layer becomes comparatively denser. Thus, the cleaning effect of the bias electric field can more effectively be attained.
When the bias application unit is provided in the transfer device in particular, a high degree of adhesion between the dielectric tube and the roller-shaped member can be obtained, and an air layer may not possibly be formed between the two. Therefore, electric nonuniformity on the cleaning member due to the air layer may not occur. Thus, imperfect cleaning of the transfer conveyor belt due to electric nonuniformity on the cleaning member can be prevented.
When the bias application unit is provided in the transfer device in particular, the roller-shaped member to which a cleaning bias is applied is covered with the dielectric tube with the extending sections, therefore, the roller-shaped member and the transfer conveyor belt do not directly contact each other even at both ends of the roller-shaped member in its axial direction. Thus, a leak of a cleaning bias due to a contact between the roller-shaped member and the transfer conveyor belt can surely be prevented.
According to still another aspect of this invention, in an image formation apparatus, by using the transfer device as a transfer device, toner dirt stuck on an image carrier which is deposited on a transfer conveyor belt or splashed toner at the time of transfer can sufficiently be cleaned off in a state where sufficient cleaning performance is delivered on the transfer conveyor belt. Thus, soil on the back of a transfer material due to imperfect cleaning of the belt, or defective transfer can be prevented from its occurrence.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.